assistant: Show error messages in popover notification (#15808)

<img width="644" alt="image"
src="https://github.com/user-attachments/assets/ee6f2e60-e50a-481e-98b4-6c4b72a9b882">


Release Notes:

- N/A

---------

Co-authored-by: Thorsten <thorsten@zed.dev>

crates/assistant/src/context.rs

@@ -281,6 +281,7 @@ impl ContextOperation {
 
 #[derive(Debug, Clone)]
 pub enum ContextEvent {
+    AssistError(String),
     MessagesEdited,
     SummaryChanged,
     EditStepsChanged,
@@ -1580,6 +1581,10 @@ impl Context {
                         .err()
                         .map(|error| error.to_string().trim().to_string());
 
+                    if let Some(error_message) = error_message.as_ref() {
+                        cx.emit(ContextEvent::AssistError(error_message.to_string()));
+                    }
+
                     this.update_metadata(assistant_message_id, cx, |metadata| {
                         if let Some(error_message) = error_message.as_ref() {
                             metadata.status =